1. Field of the Invention
The present invention relates to a wireless communication system and a wireless communication device. More particularly, it relates to a wireless communication system in which data communication is performed by a broadcast system and a multicast system under a wireless communication LAN environment, and a wireless communication device for use in the system.
2. Description of the Related Art
Wireless LAN communication is communication which is performed in accordance with a frame format and a protocol for communication control defined by the standard of IEEE802.11 stipulated by the Institute of Electrical and Electronics Engineers (hereinafter referred to as IEEE) (e.g., see IEEE standard 802.11, 1999 Edition, IEEE standard 802.11e, 2005).
As a data communication method in a medium access control (MAC) layer according to the standard of this wireless LAN communication, three types of methods are defined. That is, they are a unicast (UC) system in which data is transmitted to one specific wireless terminal, a broadcast (BC) system in which data is transmitted to all wireless terminals, and a multicast (MC) system in which data is transmitted to one or more specific wireless terminals. Among these systems, in the UC system, there is introduced a mechanism in which acknowledgment (hereinafter referred to as the ACK) is performed with respect to the data transmission. However, in the BC system or the MC system, there are not introduced a mechanism in which the acknowledgment is performed with respect to the data transmission and a mechanism of block acknowledgment (Block ACK) (hereinafter referred to as the BA) with respect to a plurality of frames.
An ACK return method to be performed by the UC system, which has heretofore been used, will hereinafter be described. On receiving a data frame, a receiver returns the ACK to a sender after a short interframe space (hereinafter referred to as the SIFS) of a data transmission interval in the wireless LAN. Here, the SIFS is set to 16 μs in the standard of IEEE802.11a, and is set to 10 μs in the standard of IEEE802.11b. Thus, the standard is determined with regard to the ACK return in the UC system. However, in the BC or MC system, the standard of the ACK return during the data transmission is not determined, and the ACK does not have to be returned.
However, the return of the ACK becomes important in a certain case, depending on the contents of the transmission data. For example, in the case of the distribution of stock price information, when stock prices have to be securely notified to all the receivers simultaneously without inequality, the data transmission and the return of the ACK by the BC or MC system are essential.
Therefore, when the ACK return method used in the UC communication is used as it is as the acknowledgment with respect to the data frame transmitted by the BC or MC system, the following problem occurs. That is, the ACK is simultaneously returned from all the receivers to the sender, and hence the frames transmitted from the plurality of receivers collide with one another and the information collapses. As a result, the sender cannot identify the receiver which has transmitted the frame. That is, the ACK return method for use in the UC communication cannot be used in the BC or MC system without being contrived.
As described above, in the frame transmission method of the BC or MC system by a protocol sequence using control and management frames defined in the standards of IEEE802.11 and IEEE802.11e, there is not present a mechanism in which the sender recognizes that the receiver has received the frame, and hence the ACK cannot be returned. Therefore, the following problems occur.
(a) Even when frame loss is generated and a receiver side cannot receive any frame, a sender side cannot recognize this fact, and processing of resending the frame cannot be performed.
(b) If the mechanism of the acknowledgment in the UC communication is simply applied to the acknowledgment of the BC or MC communication, acknowledgment frames collide with one another under a wireless LAN communication environment, and hence the acknowledgment frame cannot normally be received on the sender side.
It is to be noted that in the standard of IEEE802.11e, the format of the acknowledgment frame with respect to a plurality of frames is defined.